1. Field of the Invention
The present invention relates to an electric power converter for converting DC power to AC power and in a motor driving system employing the electric power converter. More particularly, the present invention relates to an electric power converter suitable for compensating a dead time set in a drive signal for driving a semiconductor switching device, and to a motor driving system employing the electric power converter.
2. Description of the Related Art
An electric power converter, such as an inverter, is used to convert a DC voltage to an AC voltage by switching upper and lower switching devices, which are connected in series, with pulse width modulation (PWM), for example, and to drive an AC electric motor. If the upper and lower switching devices are turned on at the same time, an overcurrent flows with short-circuiting between the upper and lower switching devices, thus resulting in damages of those switching devices. To avoid such a trouble, therefore, a short-circuiting preventive time (dead time) is generally provided to a gate signal for driving each switching device.
With the provision of the dead time, however, a final output voltage is made ineffective in amount corresponding to the dead time. The ineffective voltage has a polarity opposed to that of a current and causes a drop of the output voltage of an inverter. The drop of the output voltage causes a ripple in the motor current in a low speed range, and the current ripple generates a torque variation.
As the related art for overcoming the above-mentioned problem, JP-A-5-64457 (Patent Document 1), for example, discloses a method of setting a compensation voltage value (ΔV), which corresponds to a certain ineffective voltage value, depending on the polarity of a voltage command (V*), and adding the compensation voltage value (ΔV) to the voltage command (V*) to provide a final voltage command value. Thus, by compensating the voltage in advance, a required average voltage is ensured with no delay.
Also, JP-A-2002-218794 (Patent Document 2), for example, discloses a method of estimating a motor resistance value (R) from both a moving average value of a voltage command (V*) and a current command (I*), multiplying the estimated motor resistance value (R) by the current command (I*) to obtain a compensation voltage value (ΔV), and adding the compensation voltage value (ΔV) to the voltage command (V*) to provide a final voltage command value.